Tungsten doped FeCoP₂ nanoparticles embedded into carbon for highly efficient oxygen evolution reaction

Abstract

Designing active and stable electrocatalysts with economic efficiency for oxygen evolution reaction (OER) is essential for developing water splitting process at an industrial scale. Herein, we rationally designed a tungsten doped iron cobalt phosphide incorporated with carbon (W_x–FeCoP₂/C), prepared by a mechanochemical approach. X-ray photoelectron spectroscopy (XPS) revealed that the doping of W led to an increasing of Co³⁺/Co²⁺ and Fe³⁺/Fe²⁺ molar ratios, which contributed to the enhanced OER performance. As a result, a current density of 10 mA cm⁻² was achieved in 1 M KOH at an overpotential of 264 mV on the optimized W_0.1–FeCoP₂/C. Moreover, at high current density of 100 mA cm⁻², the overpotential value was 310 mV, and the corresponding Tafel slope was measured to be 48.5 mV dec⁻¹, placing it among the best phosphide-based catalysts for OER. This work is expected to enlighten the design strategy of highly efficient phosphide-based OER catalysts.

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1. Introduction

The energy and environmental crisis has led to widespread concern regarding sustainable and clean energy. With the advantages of high mass-specific energy density (120 MJ kg⁻¹), environmental friendliness and renewability, hydrogen (H₂) is considered as a promising energy carrier that can substitute the fossil fuels.¹ Currently, nearly 95% of H₂ production is still derived from the hydrocarbon cracking which is of high cost and low purity.² Water electrolysis is one of the most efficient methods to produce H₂ efficiently and sustainably, but encounters inevitable dynamic overpotentials associated with hydrogen and oxygen evolution reactions (HER and OER). However, OER is a rate-limiting step for the feasibility of the water splitting system as a sustainable technology for H₂ production, due to its four proton-coupled electron transfer, which is kinetically not favored and requires more electrical energy input.^3,4 Therefore, it is crucial to design tungsten-doped bimetallic phosphide OER catalysts to reduce the overpotential of water electrolyzers.

It is well known that RuO₂ and IrO₂ are the state-of-the-art OER electrocatalysts, but they are severely limited by their high price and scarcity. To this end, people have been engaged in searching for new materials, which including transition-metal oxides/hydroxides/oxyhydroxides,^5–13 transition metals nitrides,^14–16 transition metals carbides^17–19 and transition metals sulfides,^20–22 as highly active and abundant OER catalysts. Among them, transition metal phosphides (TMPs) have gained tremendous attention in the field of OER owing to their adjustable composition, high stability and metallic properties.^23–25 For instance, Ni₅P₄ film,²⁶ CoP nanoneedles,²⁷ and Co₂P nanoparticles²⁸ are gradually being reported for OER electrocatalytic processes with high activity and durability. However, the intrinsic OER catalytic performance of pure TMPs is not outstanding. Thus, the improvement of their OER catalytic activity is crucial for the realization of excellent OER catalysts.²⁹

On the one hand, the introduction of a secondary metal, such as NiMoP,³⁰ FeCoP,²³ and (Ni_xFe_1−x)₂P,³¹ would optimize the adsorption and desorption of key intermediates and create multiple active sites,²⁴ resulting in higher catalytic activity. On the other hand, heteroatom doping (such as V,³² Mo,³³ and W³⁴) is an effective strategy to manipulate the electronic structure and reduce the energy barrier of the OER process.³⁵ Especially, W doping has been proven to effectively enhance the activity of the catalyst by promoting the water adsorption and dissociation.^36–39 More remarkably, a further boost in catalytic activity can be achieved by the combination of TMPs with functionalized carbon materials due to the improved conductivity, robust durability, and modifiable surface chemistry,²⁹ i.e., Co_xP@N, P-doped carbon,⁴⁰ NiCoP/C nanoboxes,⁴¹ and FeP₂–NiP₂@P doped carbon.⁴²

Based on the above considerations, we reported the simple and facile synthesis of innovative composite electrocatalysts, which was described as the W-doped FeCoP₂ nanoparticles embedded in the carbon matrix, by ball milling and phosphorisation. The incorporated W and composite with carbon endow the catalysts with high crystallinity, greater conductivity, larger surface area, and good nanostructured features. As a result, the W_0.1–FeCoP₂/C electrocatalyst outperformed most of the previously reported OER TMPs-based catalysts. It required only 264 and 310 mV overpotentials to reach 10 mA cm⁻² and 100 mA cm⁻² current density for OER, respectively, with a Tafel slope of 48.5 mV dec⁻¹ and durability of 30 h measured cycles in 1.0 M KOH, attaining a high current density (100 mA cm⁻²).

2. Experimental section

2.1 Materials

All chemical reagents were analytically pure and were used as received without further purification, including C₁₅H₂₁FeO₆ (98%, Aladdin, China), Co(CH₃COO)₂·4H₂O (98%, Aladdin), WCl₆ (99%, Aladdin), NaH₂PO₂ (99%, Aladdin), NaH₂PO₂ (99%, Aladdin).

2.2 Synthesis of W_x–FeCoP₂/C electrocatalyst

Firstly, 4 mmol of C₁₅H₂₁FeO₆, 4 mmol of Co(CH₃COO)₂·4H₂O, and 0.1 mmol of WCl₆ were mixed homogeneously by ball milling.

After that, the mixture and 1.5 g NaH₂PO₂ were placed at two separate corundum boats, and the mixed precursor was at downstream in the tube furnace. Then, the tube furnace was heated at 350 °C with a heating rate of 2 °C min⁻¹ keeping for 2 h and cooled down to room temperature under the flowing N₂.

For the preparation of W_0.15–FeCoP₂/C, W_0.05–FeCoP₂/C, FeCoP₂/C, WCl₆ was added to the reactants at different amounts (59.4 mg, 19.8 mg, and 0 mg).

2.3 Materials characterization

The crystallographic patterns of the as-prepared materials were investigated by X-ray diffraction at 2° min⁻¹ (XRD, Cu-Kα irradiation; λ = 1.5418 Å). The morphology and elemental distribution of the samples were observed by a scanning electron microscopy (SEM Hitachi S-8010) and a transmission electron microscopy (TEM; FEI Tecnai G2 F20 S-TWIN) equipped with an energy dispersive X-ray detector (EDX). X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi spectrophotometer) was used to characterize the chemical state of samples. The specific surface area and pore size distribution of the samples were characterized by the Kubo-X1000 specific surface and porosity analyzer. The atomic ratio of W_x–FeCoP₂/C was measured by ICP-OES (Optima 8300DV).

2.4 Electrochemical measurements

The electrochemical measurements were carried out in a three-electrode configuration in 1 M KOH alkaline electrolyte using Ivium electrochemical analyzer. 5 mg of samples were dispersed in 1 ml of mixed solution (500 μl ethanol, 440 μl H₂O and 60 μl Nafion solution (Sigma-Aldrich, 5 wt%)). The mixture was sonicated for 30 min to form a homogeneous ink. Then 10 μl of the dispersion (containing 50 μg of sample) was loaded onto a glassy carbon electrode with 4 mm diameter (loading 0.4 mg cm⁻²). The resulting catalyst films were then dried at room temperature. The glassy carbon electrode with different catalysts served as the working electrode. A saturated calomel electrode (SCE) was used as the reference electrode and a graphite rod as the counter electrode. Prior to the tests, cyclic voltammetry (CV) was performed at 50 mV s⁻¹ from 0.2 to 0.4 V vs. SCE for 30 cycles before LSV was recorded for each sample. Linear sweep voltammetry (LSV) was performed in 1 M KOH solution with a scan rate of 5 mV s⁻¹. Electrode potentials were converted to the RHE scale using E_(RHE) = E_(measured) + 0.2415 + 0.059 × pH, and 90% IR correction was performed by using the ESR (equivalent series resistance) measured by EIS (electrochemical impedance spectroscopy).

Tafel plots, which quantify the OER performances were obtained by plotting the polarization curves as overpotential (η) versus log current (log(j)). By fitting the linear portion of the Tafel plots to the equation (η = b log(j) + a), the Tafel slope (b) can be acquired. Electrochemical impedance spectroscopy (EIS) measurements of the catalysts were performed at an overpotential of 0.4 V. The frequency range was 10⁵ Hz to 10⁻² Hz, and the amplitude of the applied voltage was 30 mV. The electrochemical surface areas (ECSA) were estimated by CV at different scan rates from 5 mV s⁻¹ to 200 mV s⁻¹. The long-term stability tests were carried out for 30 h by using the amperometric i–t curve at overpotential of 0.3 V.

3. Results and discussion

The XRD patterns of the W_0.1–FeCoP₂/C, W_0.15–FeCoP₂/C, W_0.05–FeCoP₂/C and FeCoP₂/C catalysts were displayed in Fig. 1. It can be observed that the peaks of four samples can be well indexed to FeCoP₂ phase (ICSD #622953). The characteristic peaks positioned at 31.8°, 36.9°, 46.3°, 48.2° and 56.5° corresponding to the (011), (111), (211), (103) and (321) planes of FeCoP₂, respectively. In addition, the broad peak near 25° can be ascribed to the carbon.⁴³ Moreover, no other obvious diffraction peaks were observed in the XRD patterns, which indicated that there are no impure phases. Of note, compared to the FeCoP₂/C, with the increase of W doping amount, the diffraction peaks of W_x–FeCoP₂/C shifted to lower angles. Especially, the magnified characteristic diffraction peak at 31.6° showed a subtle shift to lower angle direction as shown in Fig. 1. It may be attributed to the incorporation of W element into the FeCoP₂ lattice, resulting in the lattice expansion.⁴⁴

Fig. 1 XRD patterns of the W_0.1–FeCoP₂/C, W_0.15–FeCoP₂/C, W_0.05–FeCoP₂/C and FeCoP₂/C.

SEM characterizations were performed to explore the effect of tungsten dopant usage on the morphology of as-prepared W_x–FeCoP₂/C catalysts. As shown in Fig. S1a,† FeCoP₂/C was formed by the aggregation of nano particles (20–50 nm). Interestingly, as the doping amount of W increasing, W_x–FeCoP₂/C showed more dense particles (Fig. S1b, c† and 2a). In particular, W_0.1–FeCoP₂/C showed more uniform nano particles (Fig. 2a). This result indicated a microstructural adjustment achieved by W doping in FeCoP₂/C. Therefore, the surface area and pore size distribution of FeCoP₂/C and W_x–FeCoP₂/C were studied using BET analysis. Fig. S2† represented the N₂ adsorption–desorption curves of the type IV isotherm, which extended up to the relative pressure (P/P₀) range of 0.8–0.9 and an upward sweep near the saturation pressure confirmed the presence of a macroporous nature with multilayer adsorption (the insets of Fig. S2†).⁴⁵ And the obtained results of the surface area, macro-/mesopore volume were tabulated in Table S1.† The surface area of pristine FeCoP₂/C was 22.81 m² g⁻¹ with macro-/mesopore volume at 0.0095/0.074 cm³ g⁻¹. While, W_x–FeCoP₂/C possessed larger surface areas and pore volume than FeCoP₂/C after W doping, in which W_0.1–FeCoP₂/C exhibited a maximum BET surface area as high as 36.94 m² g⁻¹. It was universally acknowledged that a high surface area with a highly porous structure of the material facilitates the more active sites for reaction leaded to better performance of electrocatalysts toward the OER.

Fig. 2 (a) SEM image of W_0.1–FeCoP₂/C in different magnification (b) TEM image of W_0.1–FeCoP₂/C (c) high-resolution TEM image of the W_0.1–FeCoP₂/C nanostructure with a corresponding fast Fourier transform pattern (d) HADDF-TEM image of the W_0.1–FeCoP₂/C and the corresponding elemental mapping of (e) carbon (f) phosphorus (g) cobalt (h) iron and (i) tungsten.

The TEM image of W_0.1–FeCoP₂/C further confirmed the porous structure (the white circle in Fig. 2b). And the HRTEM image (Fig. 2c) of W_0.1–FeCoP₂/C showed typical lattice fringes that matched the standard FeCoP₂ (011)/(211) interplanar spacing of 2.94/2.14 Å (ICSD #622953). Furthermore, three diffraction spots in the inset SAED patterns of Fig. 2c were assigned well to the FeCoP₂ (011), (211) and (111) planes, respectively. The high-angle annular dark field scanning TEM (HAADF-STEM) (Fig. 2d) image and the corresponding elemental mappings (Fig. 2e–i) of W_0.1–FeCoP₂/C showed that C, P, Fe, Co, and W elements were uniformly distributed. The ICP-OES results in Table S2† showed that the elemental ratios of Fe and Co in W_x–FeCoP₂/C were close to the stoichiometric ratio (1 : 1), while the ratio of the actual contents of W were slightly lower than that expected from the feed molar ratio: 2.44% in the feed resulted in 2.37% W, and both 4.76 and 6.89% feed ratios resulted in 4.43 and 5.99% W in the final material.

Based on the above results, the formation process of W_x–FeCoP₂/C can be inferred as follows: the carbon source in C₁₅H₂₁FeO₆ and Co(CH₃COO)₂·4H₂O forms a three-dimensional network structure to create a small reactor during the heating process, which separates the metal elements and accommodate the phosphatization reaction at the time. As such a result of high temperature phosphatization and carbonization with space confinement effect, W doped FeCoP₂ nanoparticles were encapsulated by porous carbon substrate and evenly dispersed. To confirm the successful coating of the carbon layer, the Raman spectrum was shown in Fig. S3.† FeCoP₂/C and W_x–FeCoP₂/C display two distinct peaks at 1348 cm⁻¹ (D band) and 1607 cm⁻¹ (G band) because of the carbon layer in Table S3.† And the calculated I_D/I_G values of FeCoP₂/C and W_x–FeCoP₂/C were all exceeding, which suggested that the catalyst have many defects, and the disordered structure was beneficial to improving the hydrogen ion diffusion and electron conduction.⁴⁶

The electronic states and the surface chemical composition of W_0.1–FeCoP₂/C were determined by X-ray photoelectron spectroscopy (XPS) analysis. As illustrated in Fig. 3, the typical peaks representing Fe, Co, and P can be clearly detected in the survey XPS spectrum of W_0.1–FeCoP₂/C. Focusing on the high-resolution XPS spectrum of Fe 2p (Fig. 3a), the peaks located at 719.8 and 707.1 eV were demonstrated as the characteristic doublets of partially positively charged Fe–P bonds which were very close to the metallic state Fe⁰, while the peaks at 724.2 (728.9) and 710.8 (714.4) eV were ascribed to 2p_1/2 and 2p_3/2 for Fe²⁺ (Fe³⁺), respectively.^47–49 Similarly, the Co (2p) spectrum (Fig. 3b) can also be deconvoluted into four sets of peaks, in where the two main peaks at 793.7 and 778.7 eV were assigned to partially positively charged Co–P bonding,⁵⁰ and the strong peaks located at 797.8 (801.6) and 782.1 (784.9) eV belong to Co³⁺ (Co²⁺) 3d_3/2 and Co³⁺ (Co²⁺) 3d_5/2, respectively, while the peaks at 805.0 and 787.5 eV were assigned to satellite peaks.⁵¹ Meanwhile, in the XPS spectrum of P 2p (Fig. 3c), the peak at 134.3 eV correspond to P–O, and the other peak at the binding energies of 129.8 eV can be attributed to P in phosphide.^51–54 The Fe, Co, P spectrum of W_0.05–FeCoP₂/C, W_0.15–FeCoP₂/C and FeCoP₂/C were showed in Fig. S4–S6.† The above XPS characterization results can further prove the existence of FeCoP₂ phase, which was consistent with XRD results.

Fig. 3 (a) Fe 2p XPS spectra for W_0.1–FeCoP₂/C; (b) Co 2p XPS spectra for W_0.1–FeCoP₂/C; (c) P 2p XPS spectra for W_0.1–FeCoP₂/C; (d) Co³⁺/Co²⁺ molar ratios of FeCoP₂/C, W_0.05–FeCoP₂/C, W_0.1–FeCoP₂/C and W_0.15–FeCoP₂/C.

In addition, as demonstrated by the W 4f spectrum (Fig. S7†), two weak peaks at the binding energies of 37.9 and 35.8 eV were attributed to W 4f_7/2 and W 4f_5/2, respectively, which suggested that W was successfully doped into FeCoP₂ and mainly existed in W_x–FeCoP₂ in the form of W⁶⁺.⁵⁵ Notably, compared with FeCoP₂/C, the peak area contents of Fe–P bond in W_x–FeCoP₂ were significantly decreasing (Fig. S4†), while those of Fe³⁺ and Fe²⁺ were increasing. The above mentioned changes indicated Fe moved to the oxidation state, due to the high valence W⁶⁺ can promote the oxidation of 3d metals.⁵⁶ Interestingly, similar phenomena had also occurred in the Co 2p spectra (Fig. S5†), in where the ratio of Co³⁺ to Co²⁺ content gradually changed with the change of W doping amount.

Based on the XPS results, the molar ratio of Co³⁺ to Co²⁺ (Co³⁺/Co²⁺) in each sample is quantitatively analyzed which was shown in Fig. 3d. The results showed that Co³⁺/Co²⁺ in W_0.05–FeCoP₂/C, W_0.1–FeCoP₂/C and W_0.15–FeCoP₂/C samples were all more than 1, and the maximum Co³⁺/Co²⁺ in W_0.1–FeCoP₂/C samples was 1.62. However, Co³⁺/Co²⁺ of FeCoP₂/C sample was only 0.97, which indicated that the doping of tungsten atoms leaded to the increase of high price Co ions in the samples. Recently, many studies point out that high cobalt (Co^3+δ, 1 ≥ δ ≥ 0) was the active center of OER catalyst,^57–59 Co³⁺ showed higher activity than Co²⁺.⁵⁹ The low price of Co would oxidize to the high price (Co²⁺ → Co³⁺, Co³⁺ → Co⁴⁺) during the catalytic reaction, while this oxidation process required a lot of energy. If Co³⁺ in the catalyst can be directly increased, the reaction barrier of the process will be reduced, so as to effectively improve the activity of the catalyst.^60,61

The OER electrocatalytic properties of the prepared W_0.1–FeCoP₂/C, W_0.15–FeCoP₂/C, W_0.05–FeCoP₂/C and FeCoP₂/C catalyst were measured in 1 M KOH electrolyte at room temperature. All of the potentials were calibrated against the reversible hydrogen electrode (RHE) for comparison. Fig. 4a showed the polarization curves of W_0.1–FeCoP₂/C, W_0.15–FeCoP₂/C, W_0.05–FeCoP₂/C and FeCoP₂/C catalyst. W_0.1–FeCoP₂/C demonstrated a much lower onset potential and higher specific current compared to W_0.15–FeCoP₂/C, W_0.05–FeCoP₂/C and FeCoP₂/C, revealing that it had excellent intrinsic OER catalytic activity. As Fig. 4c compared, to achieve J = 10 mA cm⁻², W_0.1–FeCoP₂/C required an overpotential of 264 mV, which was significantly lower than that of W_0.15–FeCoP₂/C (286 mV), W_0.05–FeCoP₂/C (325 mV), and FeCoP₂/C (389 mV). Since the current density of W_0.1–FeCoP₂/C achieved 100 mA cm⁻², the overpotential was 310 mV which was far below FeCoP₂/C (524 mV). The results indicated that sufficient doping of W atoms made catalysts behave superior to the IrO₂. The performance of W_0.1–FeCoP₂/C was superior to those of most previously reported nanomaterials (Table S4†). The Tafel slope for the W_0.1–FeCoP₂/C catalyst was 48.5 mV dec⁻¹ in Fig. 4b, confirming the OER reaction kinetics. With values of 65.5 mV dec⁻¹ for W_0.15–FeCoP₂/C, 90.6 mV dec⁻¹ for W_0.05–FeCoP₂/C, and 154.3 mV dec⁻¹ for FeCoP₂/C, the other catalysts had much higher Tafel slopes, indicating a lower conversion efficiency in water oxidation. The electrochemical surface areas (ECSA) were measured and compared in Fig. 4d. With measuring the capacitive current the C_dl was related to double-layer charging from the scan-rate dependence of cyclic voltammetry stripping. The corresponding CV plots at scan rates from 10 mV s⁻¹ to 200 mV s⁻¹, the C_dl of W_0.1–FeCoP₂/C was 7.15 mF cm⁻² which was the highest among that of the W_0.15–FeCoP₂/C (6.78 mF cm⁻²), W_0.05–FeCoP₂/C (6.25 mF cm⁻²), and FeCoP₂/C (5.78 mF cm⁻²). It coincided well with the polarization and Tafel slope curves. This was due to the phosphidation process, which would generate more porous structure as reported in literature. Due to the similar surface morphology of four catalysts, the values of C_dl were very similar (Table S5†). The Nyquist plots were used to analyze the charge transfer resistances in Fig. 4e. The inset of Fig. 4e showed the fitted equivalent circuit, in which R_s and R_p represented the solution resistance and charge-transfer resistance, respectively. The solution resistance was consistent for all the measurements with a value of 6 Ω. W_0.1–FeCoP₂/C showed a much lower interfacial charge transfer resistance of 7.86 Ω as compared to the others, which was consistent with their OER activities (FeCoP₂/C < W_0.05–FeCoP₂/C < W_0.15–FeCoP₂/C < W_0.1–FeCoP₂/C) and signified the obviously greater reaction kinetics of W atom doped. Tungsten atoms possessed a large radius and entered into the phosphide lattice can increase the crystal surface defects, which leaded to the more active sites in the oxygen evolution reaction. Apart from the high activity, stability was also significant for OER catalyst. The electrochemical stability of the prepared W_0.1–FeCoP₂/C catalyst was tested by chronoamperometric (i–t) methods, as shown in Fig. 4f. The current density of W_0.1–FeCoP₂/C was recorded for 30 hours at an overpotential of 310 mV. The OER current remains 93.7% of its initial value after the test, which indicated the excellent durability of W_0.1–FeCoP₂/C. Due to carbon packed nano phosphide nanoparticles outside, W_0.1–FeCoP₂/C exposed more active sites used for OER reaction and simultaneously enhanced the stability.

Fig. 4 (a) OER polarization curves of FeCoP₂/C, W_0.05–FeCoP₂/C, W_0.1–FeCoP₂/C, W_0.15–FeCoP₂/C and IrO₂, (b) overpotential at 10 mA cm⁻² and 100 mA cm⁻² of FeCoP₂/C, W_0.05–FeCoP₂/C, W_0.1–FeCoP₂/C and W_0.15–FeCoP₂/C, (c) Tafel plots for different electrocatalysts in 1 M KOH solution, (d) plots of the different current density vs. the scan rate for different electrodes, (e) Nyquist plots for different electrocatalysts recorded at 1.53 V vs. RHE, (f) the prolonged chronoamperometric response of W_0.1–FeCoP₂/C.

In order to investigate the electron states of W_0.1–FeCoP₂/C after OER, XPS spectra were collected with respect to the pristine catalyst. As compared to the pristine catalyst in Fig. 5a and b, the Fe–P bond almost disappeared after the OER test and the Co 2p XPS spectrum after OER had shifted to the higher binding energy side, which indicated the high valence Co species were formed at the electrochemical oxidation process. Meanwhile, the absence of the P element was detected decreasing rapidly by XPS in Fig. 5c. However, as shown in Fig. 5d, the signal of W 4f can still be detected after OER reaction, and there was no significant change compared with that before OER reaction (Fig. S7a†). The XRD pattern of the W_0.1–FeCoP₂/C after OER was shown in Fig. S8,† which can be seen that there was no significant change in the overall phase of the sample. These results indicated that the surface of W_0.1–FeCoP₂/C has been in situ transformed into tungsten-doped Fe–Co bimetallic (hydrogen) oxides or hydroxy–hydroxide intermediates, in agreement with illustrated by previous literature, that the surface of metal phosphide materials will undergo oxidation in situ transformation into oxygen/hydroxide during OER process, and participate in the catalytic reaction as the active center.^62,63 The micromorphology of W_0.1–FeCoP₂/C was revealed by TEM, as Fig. 5e showed, after OER W_0.1–FeCoP₂/C still maintained the nanoparticles embedded into carbon. That can be explained the excellent stability of W_0.1–FeCoP₂/C. This can be attributed to the carbon layer coated by phosphide nanoparticles, which can effectively prevent the agglomeration and deactivation of nanoparticles during the reaction process, so that tungsten-doped Fe–Co bimetallic phosphide can maintain a long time catalytic activity in the electrochemical oxygen evolution process.

Fig. 5 The comparison of the high-resolution XPS spectra of W_0.1–FeCoP₂/C before (top) and after (bottom) OER tests: (a) Fe 2p, (b) Co 2p, and (c) P 2p; (d) XPS characterization of W 4f and (e) TEM image of W_0.1–FeCoP₂/C after OER tests.

The electrochemistry analysis, SEM, HRTEM, and X-ray analysis suggested that W_0.1–FeCoP₂/C possessed highly promising OER-type electrocatalytic properties with the following aspects. (1) Theoretical studies suggest that the energetics of OER intermediates can be modulated by incorporating metal elements, and that in turn these tune the catalytic activity of these materials. W⁶⁺ into FeCo bimetallic phosphides for more efficient OER catalysis is due to the modulation of the 3d energy levels of the metal atom, leading to good adsorption energies for OER intermediates. Fe, Co, W three elements coexist and synergistically make *OH energetics in an optimal position. (2) The optimal addition of tungsten results in the increasing of Co³⁺, which makes more electrons participate in electronic transfers for OER. (3) Phosphides are oxidized to metal oxides/hydroxides on the surface under the OER conditions in alkaline media. The metal oxides/hydroxides on the surface provide OER catalysis properties, meanwhile phosphides with metallic conductivity provide fast charge transportation. (4) The size of nano phosphide nanoparticles (W_0.1–FeCoP₂) is under 10 nm, which results in a large specific surface area and more active sites exposed. (5) The carbon outer layer also enables stabilization functionality toward the OER.

4. Conclusion

In conclusion, we presented a facile method for preparing W_0.1–FeCoP₂ nanoparticles composited with carbon which possessed outstanding OER catalytic properties. In this material, the nano phosphide nanoparticles provided abundant surface active sites and short mass transfer pathways, while the metallic conductivity facilitates charge transfer. Furthermore, the uniform distribution of Fe, Co, W three metal elements enabled adsorption energies for OER intermediates in an optimal position and lower the kinetics of OER. As expected, the W_0.1–FeCoP₂/C catalyst delivered a low overpotential of only 264 mV at a current density of 10 mA cm⁻² and a small Tafel slope (48.5 mV dec⁻¹), comparable with counterparts of different W atom (W_0.15–FeCoP₂/C, W_0.05–FeCoP₂/C and FeCoP₂/C) catalysts, and superior to most previously reported OER catalysts. After addition of the correct amount of W into Fe–Co, highest Co³⁺/Co²⁺ molar ratio leaded to greater electronic transfers, which was the important factor in boosting the OER activity. The present work stated a promising strategy to improve OER electrocatalysis through precisely tuning the composition of multimetallic phosphides.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by Grants from Subsidized Project of Suzhou Academy of Agricultural Sciences (Grant No. 22024), Agricultural Science and Technology Independent Innovation Funds of Jiangsu Province (Grant No. SCX(22)3064), Suzhou City and Tongren City Science and Technology Cooperation Project (Grant No. Tong Ke Cheng He [2020]01), the Science and Technology Foundation of Suzhou (Grant No. SNG2023002, SNG2021024).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra02326a

‡ Co-first author.

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